I'm at the tail end of two of these, of ~10 in my career. They are always tough, always a bit of chaos, and all different.
Planning is important, and avoid committing to targets or deadlines until you have your arms wrapped around what needs to be done. This can be wide-ranging, and include: product parity, contract management, internal asset development (project plans, test suites, customer training, etc.), customer change management, and team throughput.
You have few clients but large impacts. You likely want to pick the friendliest one and give them generous terms to be the "test case". Expect it will take 2x longer than your estimate.
Do as much work on parity as you can: what are the differences between v1 and v2, and how will you bridge them? If data migration is involved, you will need tooling and team training.
Inevitably you will find that customers move slower than you like and are using v1 in ways you did not expect.
Planning is important, and avoid committing to targets or deadlines until you have your arms wrapped around what needs to be done. This can be wide-ranging, and include: product parity, contract management, internal asset development (project plans, test suites, customer training, etc.), customer change management, and team throughput.
You have few clients but large impacts. You likely want to pick the friendliest one and give them generous terms to be the "test case". Expect it will take 2x longer than your estimate.
Do as much work on parity as you can: what are the differences between v1 and v2, and how will you bridge them? If data migration is involved, you will need tooling and team training.
Inevitably you will find that customers move slower than you like and are using v1 in ways you did not expect.